PRIOR ART
Processes for the oligomerization of light olefins to produce C.sub.7 and higher carbon number oligomers are well known. Oligomerization processes have been long employed to produce good quality motor fuel components from propylene and butylene. Such oligomerization processes are also referred to as catalytic condensation and polymerization with the resulting motor fuel often referred to as polymer gasoline. Methods have always been sought to improve the octane number of the gasoline boiling range oligomerization products. In addition the oligomerization process is also susceptible to catalyst fouling from the condensation of heavy oligomers into coke that covers the catalyst.
Another process that has met the continuing demand for the conversion of light hydrocarbons into high octane motor fuels was the alkylation of isobutane with propylene, butenes and amylenes using a hydrofluoric acid (HF) catalyst, commonly referred to as HF alkylation. The HF process has provided a highly successful method for the production of high octane motor fuels. Despite a long history of safe operation, recent concerns over the possibility of a catastrophic release of HF acid from HF alkylation units has prompted the investigation of modification or alternatives to the HF alkylation process for the production of motor fuels. One existing alternative is a similar alkylation process that uses sulfuric acid as the catalyst. While the use of sulfuric acid may decrease the degree of the hazard that some associate with the use of HF acid, the sulfuric acid process is still perceived as possibly presenting the same hazard and is not as economically advantageous as the HF alkylation process. Therefore, processing methods have been sought to improve the operation of oligomerization processes as substitutes for acid catalyzed alkylation.
A number of arrangements are known for using oligomerization in combination with other processes such as saturation and dehydrogenation as substitutes for acid catalyzed isomerization alkylation. Patents disclosing the dehydrogenation of light paraffin stream with oligomerization of the dehydrogenation effluent include U.S. Pat. No. 4,393,259, U.S. Pat. No. 5,049,360, U.S. Pat. No. 4,749,820, U.S. Pat. No. 4,304,948, and U.S. Pat. No. 2,526,966.
It also known to hydrotreat the olefinic hydrocarbon streams produced by oligomerization to saturate olefins. Patent GB 2186287 discloses dehydrogenation of and oligomerization of a C.sub.4 fraction to produce a jet aircraft fuel that is optionally hydrogenated into premium gasoline. The hydrotreatment of jet fuels, diesel fuels and lubes produced by dehydrogenation and oligomerization of light paraffins is disclosed in U.S. Pat. No. 4,678,645. However, hydrotreating is not always beneficial for gasoline fractions produced by oligomerization and can lower octane ratings, but is known to be particularly beneficial when saturating isooctenes to isooctanes.
Some prior art process that use highly acidic halide type catalysts for polymerization have suggested the recycle of paraffins to the polymerization zone for purposes of cooling. Such references include U.S. Pat. No. 4,200,714 and U.S. Pat. No. 4,254,294. However, heavier paraffin streams in these references are disclosed exclusively for the purposes of controlling reaction zone temperature.
It is a further object of this invention to increase the octane number of the saturated oligomerization products.
It is a yet further object of this invention to reduce the impact of catalyst fouling by coke accumulation in oligomerization processes.